This invention relates to a centrifugal separator for concentrating, dewatering, as well as for recovering heavy sedimentation components and separated water from sewage sludge, industrial wastewater, and several products in chemical and food industries by centrifugal force.
In solid/liquid separation of sludge, decanter type centrifugal separators have generally been used in the past. As indicated in FIG. 7, this separator is comprised of a bowl 1, (outer rotating cylinder) that is formed by connecting a cone 31, at the tip of a horizontally elongated straight drum section 30, and in which an inner cylinder 11 (inner rotating cylinder) is equipped with a spiral blade 12 and a screw conveyor 10 is provided to rotate at a relative speed difference with respect to the bowl 1, so that sludge processing liquid, a, is fed into bowl 1 from inner cylinder 11 to achieve solid/liquid separation by centrifugal force. Dewatered cake b, which is a heavy component separated by the sedimentation process, is scraped toward the front end of the bowl by spiral blade 12, receives further compaction and dewatering treatments in cone 31 before it is discharged out of the separator from sludge discharge holes 7, provided at the front end of the separator. The separated liquid c is discharged through the overflow process out of the separator through a discharge opening 32, provided at a rear end wall 3 of bowl 1 which is located at the opposite side. (Hereafter in this description the direction in which the centrifugal force is exerted, or the direction along which the bowl radius increases, is referred to as the xe2x80x9cdownxe2x80x9d direction; while the direction along which the bowl radius decreases is referred to the xe2x80x9cupxe2x80x9d direction)
This decanter type centrifugal separator, which stores filtered liquid in bowl 1, is characterized by a feature requiring that the cone 31 whose front end is squeezed to a small diameter up to the same level (water level) of the discharge hole 32 for the separated liquid, in order to prevent filtered liquid from being discharged through the sludge discharge hole 7 that is designed to discharge the cake, and in order to improve the dewatering effect by elevating the dewatered cake above the water level in the bowl by a cone section, called the xe2x80x9cbeachxe2x80x9d.
Although conventional centrifugal separators have been developed to concentrate or dewater crystals in the liquid phase, if the same separators are applied for the concentration or dewatering of processing items such as sludge, which has different characteristics from the former, it is necessary to provide a strong compaction effect in order to squeeze water out so that the dewatering efficiency can be improved, because the sedimentation layer of the sludge is pasty and is strongly hydrophilic. In the conventional decanter type centrifugal separators mentioned, the processing liquid, a, supplied to the center section of bowl 1 undergoes solid/liquid separation under the strong centrifugal force field (approximately 2,000 to 3,0000G) in the straight drum section 30 immediately after being supplied. Yet, in cone section 31 where the dewatered cake b is discharged, the centrifugal force is weak resulting in an increase in the moisture content, because its distance from the center of rotation (radius) is short. In fact, in the system depicted in FIG. 7, it has been observed that the moisture content becomes minimum in the d section, which is around the boundary between the straight drum section and the cone section. Moreover, it is necessary to elevate the cone section against the strong centrifugal force in order for the sedimentation layer to be discharged. Even if an attempt is made to move the sedimentation layer by the screw conveyor, the so-called co-rotation phenomenon due to friction resistance occurs when the moisture content is low, resulting in the cake becoming stagnant and unable to be discharged. Conversely, there is a tendency that only the cake having relatively high moisture content near the center of rotation of straight drum section 30 can be discharged.
Also since the dewatered cake b passes through a cone having a long slope in order to be discharged over the water level in the bowl, there is a disadvantage in that a slip is produced at this section impairing the discharge process, resulting in sludge being discharged together with separated liquid through a separated liquid discharge opening 32, and contaminating the separated liquid. In addition, since the dewatered cake to be discharged has a relatively high moisture content in the vicinity of the rotational center of straight drum section 31, in order to decrease the moisture content of the cake to be discharged, the current practice is to increase the rotational speed of bowl 1 beyond what is actually needed (approximately at 2,000 to 3,000 rpm), which requires a large amount of power.
In order to discharge the pasty sedimentation layer, which is difficult to transport by a screw conveyor, an operating condition called the xe2x80x9cnegative damxe2x80x9d or the xe2x80x9cupside overflowxe2x80x9d is used, in which the discharge opening position of the separated liquid is higher than the discharge opening of the sedimentation layer. One of such systems, for example, is the Ambler type system (U.S. Pat. No. 3,172,851, and Japanese Patent Application Kokai H6-190302), which uses the head press of the processing liquid in the bowl to assist in the discharge of the sedimentation layer.
Nevertheless, since the liquid level in the bowl is high, the sedimentation layer is still below the liquid surface even in the xe2x80x9cbeachxe2x80x9d section, there has been a problem in that the moisture content increases since the xe2x80x9cbeachxe2x80x9d having a low head press due to the centrifugal force is elevated as it is. (A strong centrifugal force is applied in the bowl, and some layer in the bowl receives a strong pressure due to the centrifugal force applied to the liquid layer or the sedimentation layer above. In this description, this pressure is called the head press.)
In addition, for the Lee type centrifugal separator, a separation plate having a slight gap with the bowl wall is provided in the vicinity of the boundary between the straight drum section and the cone section. An attempt is made to extract only the bottom sections of the sedimentation layer through this gap between the bowl wall and the separation plate.
Yet, as mentioned above, it is difficult to transport the pasty sedimentation layer having low moisture content by the screw conveyor. Since the usable head press is limited to the water level in the bowl, a special construction including a scraping-up device (Japanese Patent Application Kokai H4-59065) is needed for discharging such a layer.
One of these types is designed to supply the processing liquid through a rotating shaft of the bowl so that the separated liquid and sedimentation layer can be discharged through the rotating shaft (Japanese Patent Publication S63-31261). Although this system has an outstanding performance as a separator, there are cases in which difficulties have been encountered in discharging a dewatered cake having low moisture content.
All centrifugal separators mentioned above have their sedimentation layer discharge opening at essentially the same or higher level than the liquid level in the bowl. Even when the head press in the bowl is used for discharge, the head press of the processing liquid in the bowl is lower than the head press of the heavy solid layer; thus it is theoretically impossible to discharge the heavy solid layer only by the head press, thus it requires some type of discharging mechanism.
This invention is to solve the problems mentioned above for the decanter type centrifugal separator, in order for the conventional centrifugal separator to be able to achieve direct discharge of the sludge from the d section in which the moisture content is the lowest. With this invention, the separation process is expedited and its efficiency is improved, while the bowl speed reduction is realized leading to power saving, and simplification and size reduction of the system are realized since the cone shaped xe2x80x9cbeachxe2x80x9d section is no longer necessary.
In the centrifugal separator according to this invention, comprised of a rotating bowl with a high rotational speed and a screw conveyor that is provided within the bowl and rotates with a relative speed difference therewith, a discharge route for the dewatered cake is provided at one end wall of the bowl, and the opening of this discharge route into the bowl is provided in the vicinity of the inner perimeter wall of the bowl (herein, xe2x80x9cbowlxe2x80x9d means the section in which the processing liquid undergoes the solid/liquid separation process by centrifugal force.)
With this design, as far as the discharged cake from the discharge route is concerned, only its section having the highest compaction effect due to the head press of the centrifugal force being applied to the sediment in the sedimentation layer that was deposited at one end of the bowl can be discharged through the discharge route.
When the processing liquid is supplied to the bowl during the starting period of the centrifugal separator, it is not desirable to have the solid component be discharged immediately through the discharge hole without being concentrated and dewatered. In order for the solid component to achieve good sedimentation (to achieve a high transparency in the separated liquid), it is necessary that the solid component be subjected to centrifugal force in the bowl for a specified time period. It is, therefore, advantageous to have the liquid discharge route constructed in such a manner that the initial liquid level in the bowl can be maintained at least during the initial starting period.
Of course, during the operation, the separator may assume a condition called the downside overflow system in which the discharge opening for the separated liquid is lower than the discharge opening for the dewatered cake, or conversely it may assume a condition called an upside overflow system in which the discharge opening for the separated liquid is higher than the discharge opening for the dewatered cake. In the case of the upside overflow, the water level in the bowl, which is dependent on the height of the discharge opening for the separated liquid, is maintained by the sedimentation layer deposited along the side of the discharge route.
The discharge route mentioned above acts as a restriction that limits the quantity of the dewatered cake discharged from the sedimentation layer. In the centrifugal separator according to this invention, the dewatered cake in the discharge route is mainly pushed out by the head press resulting from the centrifugal force of the sedimentation layer that acts on the backside surface, the transport force of the screw, and in some cases by the supply pressure of the processing liquid to the bowl.
Since the discharge quantity is dependent on the discharge resistance exerted by the discharge route, and on the pressure pushing the dewatered cake out, the compaction effect on the dewatered cake as well as the discharge quantity are small when the thickness of the heavy component deposit layer deposited in the vicinity of the discharge route opening is small. Consequently, the thickness of the deposit layer in the vicinity of the discharge route opening gradually increases with the accumulation of the heavy component sedimentation that is scraped by the screw conveyor. Yet, the increase in the thickness of the deposit layer causes the pushing force to increase, resulting in an increase in the discharge quantity that overcomes the discharge resistance. Thus, the thickness of the deposit layer is kept constant by a balance between the accumulation quantity and the discharging quantity.
Since the specific weight of the sedimentation layer is greater than that of the processing fluid, the head press that can be used for the discharge will be greater than the head press of the processing fluid that is used in the conventional system. Especially for the condition in which the sedimentation layer protrudes above the liquid level due to the restriction effect that limits the discharge quantity, its head press becomes very high making the dewatered cake discharge easier. Furthermore, the compaction effect on the dewatered cake by the deposit layer maximizes resulting in the low moisture content of the discharged solid component.